The present invention relates to a fringe scanning shearing interferometer, and more particularly to a fringe scanning shearing interferometer capable of measuring flat, spherical, and aspherical surfaces with high accuracy, and measuring the wavefront aberration, the degree of eccentricity, and the focal length, for example, of an optical device.
Conventional fringe scanning shearing interferometers operate to measure the surface configuration of an object under test in the following manner:
The laser beam emitted from a laser beam source is converted by a collimator lens into a parallel beam. The parallel beam is then passed successively through a first beam splitter, a second beam splitter, and a convex lens to illuminate the object. The light beam reflected by the object travels back through the convex lens to the second beam splitter, which divides the beam into two beams in different directions. One of the split beams goes through the first beam splitter to a third beam splitter, from which the beam travels through a focusing lens and falls on a photodetector. This split beam is referred to as a wavefront under test.
The other split beam is reflected by a vibrating mirror fixed to a piezoelectric device or a magnetostrictive device which serves as a device for varying the length of an optical path. The reflected beam is directed toward the third beam splitter and then goes through the focusing lens to the photodetector. This split beam is referred to as a reference wavefront.
Therefore, interference fringes are formed on the photodetector by the wavefront under test and the reference wavefront. The surface configuration of the object under test can now be identified by measuring the differences between phases at points on the interference fringes and computing the wavefront under test based on the measured phase differences.
The prior interferometers have been disadvantageous in that since the wavefronts of the split beams are required to travel over long distances through many different optical elements, the wavefronts are subjected to vibrations or other disturbances of external origin, and hence the measuring accuracy remains low.